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From "Bharath Mundlapudi (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-988) saveNamespace can corrupt edits log, apparently due to race conditions
Date Wed, 01 Jun 2011 21:02:47 GMT

    [ https://issues.apache.org/jira/browse/HDFS-988?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13042432#comment-13042432
] 

Bharath Mundlapudi commented on HDFS-988:
-----------------------------------------

I am just wondering, if we are calling os sync at all on this code path. All i see is flush
call which flushes from EditLogOutputStream (java buffers) to kernel buffers.  

Shouldn't we be doing the following?

eStream.flush();
eStream.getFileOutputStream().getFD().sync();

This will make sure the edits are actually written to disk. Is there any reason for not doing
this? 


> saveNamespace can corrupt edits log, apparently due to race conditions
> ----------------------------------------------------------------------
>
>                 Key: HDFS-988
>                 URL: https://issues.apache.org/jira/browse/HDFS-988
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: name-node
>    Affects Versions: 0.20-append, 0.21.0, 0.22.0
>            Reporter: dhruba borthakur
>            Assignee: Eli Collins
>            Priority: Blocker
>             Fix For: 0.20-append, 0.22.0
>
>         Attachments: HDFS-988_fix_synchs.patch, hdfs-988-2.patch, hdfs-988-3.patch, hdfs-988-4.patch,
hdfs-988.txt, saveNamespace.txt, saveNamespace_20-append.patch
>
>
> The adminstrator puts the namenode is safemode and then issues the savenamespace command.
This can corrupt the edits log. The problem is that  when the NN enters safemode, there could
still be pending logSycs occuring from other threads. Now, the saveNamespace command, when
executed, would save a edits log with partial writes. I have seen this happen on 0.20.
> https://issues.apache.org/jira/browse/HDFS-909?focusedCommentId=12828853&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12828853

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